Obscured by Clouds. The rough and ready blog of a cloud benighted biologist and amateur astronomer. Astroblog will cover my interests in astronomy, biology and Life, the Universe and Everything.

Friday, August 08, 2014

The "Super Moon" of Sunday August 10/ Monday August 11, What Can You See?

Illustration of the orbit of the Moon around the Earth. Distances for apogee and perigee are given for the 2011 Full Moons because I was lazy and didn't want to redraw the diagram.

The Full Moon of this Monday August 11 occurs at perigee.
Actually, it is a little tricky. Full Moon is 18:09 August 10 UT, which is 04:09 August 11 AEST (03:39 ACST).

Yes, the Full Moon and perigee occurs in the early morning of August 11, if you wait until Moon rise on the evening of the 11th, it will all be over.

If you don't feel like staying up until 3am, then around 10-11 pm on Sunday the 10th will do just fine (see below).

Some folks have started to call perigeean Full Moons "Super Moons" for reasons that, to my mind, are not entirely justified. Still, we have the name, let's move on. What will you see, and what are the implications of this coming "Super Moon".

The
Moon has an elliptical orbit around the Earth (greatly exaggerated for
illustrative purposes in the diagram above). When the Moon is
closest to Earth, it is at perigee, and furthest, at apogee. The orbit
of the Moon precesses around the Earth, so that sometimes perigee occurs
at full Moon, sometimes at new Moon, and every time in between. Also
for a variety of reasons the distances of closest and furthest approach
can vary by up to almost 1,000 Km.

Perigee Full Moons ("super moons") are closer and brighter than other full Moons.

So, what can you see?

Without a telescope, and a near photographic memory, not much. This years perigee full Moon
could appear up around 17% bigger and around 30% brighter in the
sky than this years apogee Moon.

However, the full Moon is only around half a finger-width wide in the sky, 17% of half a finger-width is not very much.

Technically the Moon is around 33 arc minutes wide (33'). The limit of distances that someone
with good vision can distinguish between is 1 minute of arc (about the
width of a human hair). So, for the vast majority of people any
difference smaller than 1 minute of arc cannot be seen.

Comparison of the January 16 mini Moon and the August 11 "Super" Moon simulated in Stellarium. With the unaided eye, the Moon
only appears half a finger-width wide, so the difference is much harder
to see.

On Jan 16th, the Moon was 406536 Km from Earth at furthest remove, while
on August 11 it will be 356896 Km away at closest approach at 3 am.

At 11 am on the 10th (a more reasonable hour) the Moon will be 356926.2 Km away.

While the effect is really obvious in a telescope, visually it is very
hard to see the difference even if you have fantastic eyesight. On
January the 16th 2014, the apogee diameter was 29'32", for the perigee
Moon of August 11 at 3 am it will be 33'90". At the more reasonable hour of 11 pm on the 10th it will be 33'56".

In both cases the Moon is around half
the width of your finger, and 4' (that's minutes of arc, about 4 human
hairs in width) different. This is around the limit of what humans can
distinguish. If you have great eyesight and and a great memory you will
be able to distinguish between the January and August full Moons, but otherwise, no.

However, it
will be a good photo opportunity, if you have a decent zoom on your
camera or access to a small telescope. If you have not already taken an image on January 16 (see here for mine), taking a photo of the Moon on August 10 and then again on March 5th in 2015 you will see a decent difference (you need to use exactly the
same
zoom enlargement, see Inconstant Moon for instructions).

For a list of full/new Moons and the dates of apogee/perigee see here.

What will Happen?

The Moon will be bright and lovely, if lucky you may see a satellite pass or a meteor (sadly, there appear to be no International Space Station passes or Iridium flares at this time). Maybe you may see a Moon Bow. But nothing else will happen.

I used to believe that the effect would be inconspicuous to the naked eye until I found out in 2011 that it is actually quite obvious: http://earthsky.org/space/can-you-discern-supermoons-large-size-with-the-eye-an-observer-says-yes - the reason, I guess, it that the Moon's area in the sky changes with the square of the diameter. Wish one could experiment with that under controlled conditions ...

G'Day Daniel, I've been watching super (and mini ) Moons ever since 2011, and have never been able to tell the difference, despite being a regular observer of the Moon. Of course, differences in visual ability and memory will play a role (if you can't remember what the full Moon looked like 7 months previous you will have difficulty). I'll add your link to the main article though.